In the construction disclosed in U.S. Pat. No. 3,429,092, dated Feb. 25, 1969, a threaded connector is used to interconnect adjacent columns. In application Ser. No. 744,690, filed Nov. 24, 1976 and assigned to the assignee of this application, the threaded connector of said patent is replaced with a collar-spindle connector which eliminates reliance of interthread engagement for transferring vertical loads from one column to another.